By analyzing ice cores from Greenland and Antarctica, a research team at Sweden’s Lund University discovered

Choice B is the best answer. The convention being tested is the coordination of main clauses within a sentence. This choice correctly uses a comma and the coordinating conjunction "but" to join the first main clause ("Scientists...time") and the second main clause ("the Lund...otherwise").

Choice A is incorrect because when coordinating two longer main clauses such as these, it’s conventional to use a comma before the coordinating conjunction. Choice C is incorrect because it results in a comma splice. Without a conjunction following it, a comma can’t be used in this way to join two main clauses. Choice D is incorrect because it results in a run-on sentence. The two main clauses are fused without punctuation and/or a conjunction.